Green Manure Crops

Green Manure is a crop sown purely for the purpose of digging it in while it is still fresh and green to increase the fertility of the soil. Various crops are suitable, the commonest being mustard, rye, spinach and tares. If it is desired to add nitrogen in large quantities, then peas, tares, clover, or lupins are sometimes used. It is important to dig the crops in before they get to the flowering stage. They not only add humus to the soil, but help to retain the soluble plant foods which were present when the crop was sown. Such foods may easily be washed away during a rainy period.

Besides recycling nutrients in the soil for plant uptake, green manure crops used in rotation with other crops help reduce plant diseases and insects in the garden. Organic matter also helps improve the water holding capacity of sandy soils while aerating clay soils.

Directly the ground is free of a productive crop, one of these green manure plants can be sown instead of leaving the land bare. Spinach and rye are often sown for green manure in September or October; mustard and tares are often sown in April. Mustard has the advantage that it may be dug in less than eight weeks after sowing; but it has the disadvantage that it is liable to get club root, and so keep the disease going in the ground.

It often helps to topdress green manure crops with a little livestock manure or nitrogen fertiliser to help them get started. The crop may go dormant in the winter but will remain relatively green. This helps stop the soil being blown or washed away by wind or rain.

Turn green manure crops under by digging or using a rotavator about 4 to 6 weeks before planting the summer garden. A little extra nitrogen fertiliser will help micro-organisms in the soil break down plant material into humus. Since nutrients in the humus are in organic forms, they are released slowly to plants over time.

It is a good practice to have some suitable "green manure" seed available at all times so that directly a patch of ground is cleared of crops, and is not required definitely for anything else, the green manure crop can be sown and the resultant crop dug in. Green manure crops are a good substitute for farmyard manure which the gardener can incorporate quite easily.


Green Manure

Sow Growing period Suitable soil types Nitrogen fixer Other info

Winter tares

Jul - Sep

up to Oct, or over winter

heavy, not too acid




Mar - Sep

2 - 8 weeks

moist, fertile


can suffer clubroot

Hungarian grazing rye

Aug - Nov

autumn - spring




Alsike clover

Apr - Aug

few months to 2 years

damp, acid


short-term perennial

Field beans

Sep - Nov

over winter

damp, heavy


turn under before flowering


Apr - Aug

2 - 3 months

tolerates poor


not hardy



autumn - spring

not too heavy


not hardy everywhere

Red clover

Early Aug

anytime, useful long term

avoid acid, add lime


sow thickly, cut and mulch regularly


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